The present invention relates to a novel hydrous composite oxide of cerium and phosphorus and a method for the immobilization of strontium ions in a solution. More particularly, the invention relates to a novel hydrous composite oxide of cerium and phosphorus which can be used as a chemisorptive adsorbent for the immobilization of strontium ions in an aqueous solution with high efficiency at a relatively low temperature under hydrothermal conditions as well as to a method for the immobilization of strontium ions using the same as an adsorbent.
One of the very serious issues in the atomic industry is disposal of high-level radioactive waste materials coming from spent nuclear fuels after use in atomic power plants, the amount thereof being under progressive increase year by year. Among the various methods for the disposal of the radioactive wastes heretofore proposed, one of the promising methods is to convert the radioactive waste into a vitrified solid mass which is embedded deep in a stable stratum of ground so as to be isolated from the biosphere. A problem in this method is that, since some of the inorganic radioactive nuclides such as strontium, cesium and the like have high solubility in water in the form of ions, these nuclides contained in the vitrified radioactive waste material are always under a risk of leaching out from the vitrified solid mass when intrusion of the underground water takes place into the stratum where the vitrified radioactive waste material is embedded to cause radioactive contamination of the biosphere.
In this regard, a countermeasure is under consideration in which the vitrified mass of radioactive waste materials containing the nuclides of strontium, cesium and the like and embedded in a stratum of ground is surrounded by a barrier of solid materials having activity as an adsorbent or as an immobilizing agent for the ions of strontium, cesium and the like in order to prevent infiltration of these radioactive ions through the strata. It is reported in Japanese Patent Kokai 3-293598 and 4-143700 that certain bentonite materials of which the principal constituent is smectite have high activity for the immobilization of cesium ions in an aqueous solution but these bentonitc materials cannot be an immobilizing agent for strontium ions due to their low immobilizing activity for strontium ions.
It is also reported in Chemistry Letters, page 957 (1981) that crystalline titanate fibers have high immobilizing activity for strontium ions. The crystalline titanate fibers, however, have a disadvantage in respect of safety and from the economical standpoint because the fibers cannot be imparted with full immobilizing activity for strontium ions unless they are subjected to a heat treatment at a very high temperature of 1000.degree. C. or even higher.
In view of the above described situations in the matter of disposal of radioactive waste materials, the inventors have conducted extensive investigations with an object to discover an adsorbent having high immobilizing activity for strontium ions but free from the above described problems and disadvantages in the prior art materials including a wide variety of synthetic works to uncover such an effective adsorbent material as well as a method for the immobilization of strontium ions in a solution by using the adsorbent.